The idea of this book comes from the observation
that many works about modelling within the social sciences do not use
existing specification languages (like DEVS and UML) to describe
their models. The argument for using such a common language is to
understand the models better and to understand their structure in the
way that it is currently done in other fields like robotics and
ecology. The debatable reason the author gives for the current state
of affairs in social science is that there is actually no correct
reference model to express every model of human behaviour. Professor
Schmidt proposes the PECS agent architecture to fill the gap. He
argues that the (widely used) BDI architecture can't be considered as
a good reference model because it uses rational decision-making in
agents as an assumption. Every new framework brings something
interesting to the community in the sense that it is another point of
view about social systems. Professor Schmidt's book falls into this
category of original work that specifies a new framework for
modelling human behaviour. This framework is called PECS which stands
for Physical Conditions, Emotional State,
Cognitive Capabilities and Social Status. The present
work can thus be seen as an handbook for this reference model.

Chapter 1 presents a brief introduction to the
different uses of "agent" technology which correctly situates the
book in the existing literature. The author summarises the uses of
agents into three categories (Empirical Science, Engineering Science
and Theory) and relates each category to general results and
requirements. He situates his own efforts in the Empirical Science
field, aiming to produce reference models that require structural
similarity with real systems. He then proposes PECS as a reference
model for the modelling of social systems.

In Chapter 2, the author discusses general issues
about modelling, such as the relation with gathered data, model
design and model validation. References are unfortunately lacking but
we can find well-known issues about the bias of modellers with
respect to the systems they observe. This recovers the definition of
modelling as an interaction between two dynamical adaptive systems
given in Gell-Mann (1994). The author's
position about the definition of a "model", presented only as an
abstraction of the real system, fails by not taking into account that
modelling is a contextual activity. Minsky (1968) notes on the same subject: "To an
observer B, an object A* is a model of an object A to the extent that
B can use A* to answer questions that interest him about A". However,
building a reference model such as PECS requires abstracting from the
context to obtain a generic meta-model. An interesting part of the
book is devoted to defining a good modelling process that may be
applied more often within the field. The modelling process defined
describes the conceptual model design, involving the identification
of the model components and the description of the model structure;
the model specification (using the PECS reference model); its
implementation and its validation by comparing model outputs to data
gathered from the observed system. Even if modelling steps like model
verification and calibration are not taken into account, it is still
a great improvement to apply the techniques of software engineering
to modelling activity. Concluding this chapter, the author describes
the use of both emergence and reduction in modelling. When modelling
individual properties and behaviours, emergence is defined as the
phenomenon that leads from individual properties to group formation
and dynamics. Reduction is described as the inverse process, linking
group properties to individual behaviours and can be used as a
valuable means of validation. As the author observes: "If reduction
can be done, this is a significant indication that the model is
correct".

In Chapter 3, the author justifies his claim that
human behaviour can be modelled. Unfortunately, the beginning of the
chapter is a little bit confusing. There appears to be a conflation
between the fact that one can model the understanding of a human
being on a machine and the machine's ability to understand. Another
conflation arises in discussing the agent architecture used (PECS) -
which may be able to serve as support to every model of human beings
- and the underlying wish to create a model of human beings in the
first place. Hopefully the paragraph explaining the differences
between a model and a replica clearly and humbly defines what a model
is and what its limitations are. A replica is defined as "...an
identical copy of an original..." and a model as "...an abbreviated
depiction of an excerpt of reality based on abstraction and
idealisation... It does not claim to be a replica."

In Chapter 4, the author summarises the
behavioural models he takes as references. Each of these can be seen
either as whole models or as modules that can be co-ordinated to
represent a more complex human behaviour. Retaining a classical
classification from multi-agent systems, Schmidt sorts behaviour into
two major types: reactive and deliberative. The reactive behaviours
are categorised into four types. Firstly, instinctive behaviour that
follows a simple physical stimulus, state, reaction pattern
(reflexes). Secondly, learned behaviour that stands for instinctive
behaviour within a social context ("drive on the right"). Thirdly,
drive-controlled behaviour that is a reactive behaviour triggered by
a physical need (looking for food). Finally, emotionally controlled
behaviour that is a reactive behaviour triggered by an emotional
state. The deliberative behaviours are defined by goals that the
organism tries to achieve by elaborating action plans. They are
themselves sub-categorised into two types: constructive behaviour for
which the goal is known but cannot be changed and reflective
behaviour for which the organism has the possibility of
self-regulation. The organism can then set its own goals or it can
modify existing goals. A paragraph is dedicated to the discussion of
motives corresponding to drives, emotions or acts of will that are
associated with precedent behaviours. The level of motives for each
behaviour enables to sort the set of possible actions into a
dynamical subsumption architecture (Brooks and
Connell 1986) that creates the model dynamics.

Chapters 5 and 6 briefly describe the PECS
reference model. A more complete description is provided by Urban (2000). As already mentioned, PECS stands for
Physical Conditions, Emotional State, Cognitive
Capabilities and Social Status. These are the four main building
blocks of a particular PECS agent architecture. Adding a
Sensor-Perception module and a Behaviour-Actor module, the structure
of this agent is described by the author as able to express any model
of human behaviour and also as able to replace the
Belief-Desire-Intention architecture. It is not possible to enter
into a debate on this claim here but it seems to me that even if you
are not obliged to define each of these four building blocks for a
PECS agent, it is a very massive and static framework. The examples
given in the book seem to confirm this intuition. The two rather
simple models presented are a little bit difficult to describe using
the PECS formalism.

Chapters 7 and 8 give two examples of simple
models using the PECS specification to illustrate how one might build
models with PECS. The first one, the Adam model is very
similar to Sugarscape (Epstein and Axtell
1996) and enables the reader to familiarise themselves with
physical, emotional and cognitive PECS components. Within the Adam
model, a single agent (Adam) exists on a grid environment with food
cells and danger cells. Adam builds a cognitive map of its
environment to remember food cells to eat and danger cells to avoid.
The physical component contains only energy management (energy grows
when Adam eats food, decreases a little when he walks or a lot when
he faces a danger). The emotional component manages Adam's fear when
it faces danger. The cognitive component manages the building of the
map. The behaviour is chosen while taking into account the motives
and states of the agent: food needs, fear and so on.

The other example is the Learning Group
Model. It illustrates the use of the social status component of
the PECS reference model. Within this model, agents are students that
have choice to learn alone or together in a number of groups. If they
learn in a group, they can choose which group to learn with and apply
for it. Agents are defined both by their intelligence (ability to
learn) and by their social make-up (the ability to join a group and
need for social contacts). Their goal is to acquire the most
knowledge they can by learning. The balance between their motives
(social satisfaction and learning) and the groups' admission policies
- based on such considerations as whether the new member bring
learning abilities to the group - drives the dynamics of the
model.

I would say that both examples explain the PECS
specification and how it can be used to build models in a satisfying
way.

The final chapter is very frustrating because it
purports to describe a "real" model built with PECS. This model seems
so interesting that it is very disappointing not to have anything
more than an outline. The model described briefly is the Role-Play
Model. It aims to model the role-play of children - Mummies and
Daddies, Doctors and Nurses, Cowboys and Indians and so on - and the
different group formation, interaction and disbanding processes.
However, the author only presents features of the real system, some
ideas about the modelling process and about the structure of the
model (individual interactions) and states that it mixes the Adam
model and the Learning Group Model. This failure to deliver on
earlier claims is very disappointing. However in this last chapter,
the author also presents an interesting experiment, perhaps inspired
by "Big Brother", which is to place children in a room to play and
record them with movie cameras. This enables researchers to examine
the behaviours under study in detail afterwards.

The overwhelming impression about this book is
that it would have benefited from more rigorous completion. The
initial chapters, a rather general description of agent technologies,
modelling process and modelling of human beings, surely suffer from a
lack of references. Existing works and alternative ways of thinking
are disregarded, and there is thus no comparison to sustain the
arguments presented. In the introduction, the author claims that PECS
may replace theBDI architecture but there is a clear lack of argument
for this claim. However the book constitutes a good handbook for the
PECS specification. It may prove helpful to anyone who wants to
review diverse specifications for conceptual modelling, but will
probably have no more theoretical impact.

References

BROOKS R. and J. H.
Connell 1986. Asynchronous distributed control system for a mobile
robot. In Proceedings of SPIE's Cambridge Symposium on Optical and
Optoelectronic Engineering, Cambridge, MA.